1. Field of the Invention
The present invention relates to a printing method and apparatus, e.g., a color printer, for printing an image on a printing medium.
2. Description of the Related Art
Conventionally, printing apparatuses designed to print images on printing paper and/or cloth by discharging ink from nozzles of printing elements of printheads have been known. Efforts have been made to improve image quality and increase printing speed by various methods.
As such a method, a technique of implementing high-speed printing by reciprocal printing with two-stage printheads arranged in the print media convey direction and improving image quality by using sequential multiscan (SMS) has been proposed (Japanese Patent Laid-Open No. 9-70990).
According to the above prior art, each of the two-stage printheads arranged in the print media convey direction is scanned once to form 1-line pixels (this operation will be referred to as 2-pass printing hereinafter). As shown in FIG. 22, therefore, there are four types of half bands (half of the head size) on printing paper, namely half band 1 printed by forward scanning of the first and second heads, half band 2 printed by forward scanning of the first head and backward scanning of the second head, half band 3 printed by backward scanning of the first and second heads, and half band 4 printed by backward scanning of the first head and forward scanning of the second head.
Printing is repeated in a cycle of a set of these four types of half bands, i.e., a total of two bands.
In general, landing states (landing positions, dot sizes, dot shapes, and the like) of dots in forward printing and backward printing differ from each other due to satellites or subdroplets from the heads. The above four types of half bands therefore have their individual characteristics in printing. Since nozzles (four nozzles) for printing half bands are repeatedly combined in units of two bands, half band irregularity occurs.
In addition, to improve image quality and increase printing speed, a plurality of (e.g., two) identical printheads may be disposed in the moving direction of the carriage, and one line (band) may be formed by using a plurality of printheads.
Referring to FIG. 23, a 1-line image is formed by 2-pass printing, i.e., reciprocally scanning each of two printheads, spaced apart from each other in the carriage moving direction. In this case, two types of single bands (each corresponding to the head size) are present on printing paper, namely band 1 printed by forward scanning of the first head and forward scanning of the second head, and band 2 printed by backward scanning of the first head and forward scanning of the second head.
Printing is repeated in cycles of one pair of these two types of single bands, i.e., a total of two bands.
In the above case, since landing states of dots in forward printing and backward printing differ from each other due to satellite dots or the like from the printheads, the above two types of bands have their individual characteristics in printing. When combinations (two combinations) of nozzles for printing 1-band images in units of two bands are repeated, 1-band irregularity occurs.